WASHINGTON (CNN) -- President Bush announced Thursday that Agriculture Secretary Mike Johanns will resign his post. Johanns is expected to seek an open seat in the U.S. Senate from his home state of Nebraska.

Agriculture Secretary Mike Johanns, left, appears with President Bush at the White House on Thursday.

"Mike came by the White House here a couple of days ago and he asked for my advice," Bush said. "During the discussion, he told me that he was interested in continuing to serve America, just in a different role."

Johanns made no announcement about what that role might be.

But a Republican source said Johanns, who was Nebraska's governor for six years before coming to Washington, has told friends he will run next year for the GOP nomination for the seat being vacated by fellow Republican, Sen. Chuck Hagel, who said this month that he won't seek re-election to a third term.

Hagel said Wednesday that he had spoken with Johanns the day before and encouraged him to run for the seat. Hagel would not confirm Johanns' candidacy but said the agriculture secretary would make an announcement soon.

"If it's Mike's decision and Nebraska's choice, he would make an outstanding member of the United States Senate," Bush said. "There is no doubt in my mind."

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Bush called Johanns an outstanding member of his Cabinet and thanked him for serving his country.

"He is not only a decent person and an honest person, but he's a person who can get some things done," Bush said in brief remarks in the White House Rose Garden.

"The decision to leave this post has not been an easy one," Johanns said, speaking beside Bush. "I grew up with farmers and ranchers as my childhood heroes and my mentors. Representing them in Washington has been a great privilege."

Hagel's retirement has created an opening for Democrats as the two parties struggle for control of the closely divided Senate. Despite Nebraska's generally strong Republican leanings, Democrats have won nine out of the last 11 Senate elections in the Cornhusker State.

The popular Johanns, who was re-elected in 2002 with 69 percent of the vote, is considered a strong contender to keep the seat in GOP hands. However, he will face a primary battle against a field that includes state Attorney General Jon Bruning and Hal Daub, a former congressman and Omaha mayor.

On the Democratic side of the ballot, former Sen. Bob Kerrey, who retired in 2001 and is president of the New School in New York, has said he is considering a Senate comeback.

Hagel's encouragement of Johanns' candidacy will put an interesting twist on the race against Bruning, who announced this year that he would challenge Hagel if the senator ran for re-election. Bruning had criticized Hagel for his strong opposition to the Iraq war and support for an immigration bill unpopular with some in the GOP base.

Johanns, 57, a former mayor of Lincoln, was first elected governor in 1998. He resigned after Bush picked him to head the Agriculture Department in a Cabinet reshuffle following the president's 2004 re-election.

With the Senate divided 51-49 in favor of Democrats, Republicans need a net gain of two seats to take outright control in 2008. However, the GOP must defend 22 seats, compared with 12 for the Democrats.

Complicating Republican hopes are the departures of Hagel and three other Republican incumbents in Idaho, Colorado and Virginia. While Idaho hasn't elected a Democratic senator in more than 30 years, the seats in Colorado and Virginia are expected to be highly competitive.

In addition, two Republican incumbents -- Sens. John Sununu of New Hampshire and Norm Coleman of Minnesota -- are facing fierce battles next year in states where the Iraq war has sapped GOP popularity.E-mail to a friend

CNN's Dana Bash, John King and Suzanne Malveaux contributed to this report.